Numbers of nuclei in different tissue compartments of fetal ventricular myocardium from 16 to 35 weeks of gestation.

The aim of this study was to examine mechanisms of growth in different tissue compartments of the ventricular myocardium of prenatal human hearts. To this end, stereological methods were applied in order to estimate tissue volumes and total numbers of myocyte, connective tissue and endothelial nuclei in hearts collected after death at between 16 and 35 weeks of gestation. Volumes of tissue compartments were obtained after multiplying volume densities (estimated by test-point counting) by ventricular volumes (estimated from ventricular mass and tissue density). Absolute numbers of nuclei were calculated in similar fashion from corresponding nuclear packing densities (estimated using physical disectors). The volumes of all three tissue compartments increased linearly over the period of gestation examined, and in each case, the increase in tissue volume appeared to be due entirely to proliferation. Numbers of all three types of nuclei increased linearly whilst tissue volumes per nucleus remained constant. The net rate of production of myocyte nuclei was 35 x 10(7) per week (2.1 million nuclei per hour). The net rate of production of connective tissue nuclei was 12 x 10(7) per week (0.7 million nuclei per hour) and that for endothelial cell nuclei was 5.1 x 10(7) per week (0.3 million nuclei per hour). Predictions are made about the postnatal ages at which adult ratios of different nuclear types might be attained.